Substances to be coated such as automobiles are coated usually by applying an electrodeposition coating, an intermediate coating, a base coating and a clear coating one after another to form a multilayer coating film. In coating used in such coating steps, a technology of adding an internally crosslinked fine resin particle for the purpose of improving a rust-preventive property and controlling viscosity receives attention.
A water-borne coating composition containing such an internally crosslinked fine resin particle is usually produced by mixing water dispersion of the internally crosslinked fine resin particle and a resin emulsion produced besides. Accordingly, the internally crosslinked fine resin particle exists as resin particle distinct from resin components in a coating in the water-borne coating composition. In the coating film formed from such a water-borne coating composition, the internally crosslinked fine resin particle may not adequately disperse in a resin phase in a process of forming a coating film and effects resulting from the internally crosslinked fine resin particle may not be sufficiently exhibited.
On the one hand, in Japanese Kokai Publication Hei-4-279679, there is disclosed a coating containing crosslinked resin particles. However, here, crosslinked particles are not necessarily encapsulated in emulsion particles with reliability. There is also a possibility that it is encapsulated.
In order to improve such issues, it is conceivable that by allowing the internally crosslinked fine resin particle to exist in an oil phase of a resin emulsion in the water-borne coating, the internally crosslinked fine resin particle is highly dispersed in a coating film in the process of forming a film and functions, which the internally crosslinked fine resin particle has, is more efficiently exhibited, but there has been no method of allowing the internally crosslinked fine resin particle to exist in an emulsion particle with reliability. In the present invention, encapsulating with reliability allows effects resulting from the crosslinked particles to be sufficiently exhibited.
On the other hand, cation electrodeposition coatings are widely used as an undercoating having an anticorrosive function because of the workability of coating, which permits coating in any configuration and its safety. Particularly, a cation electrodeposition coating, in which a substance to be coated itself is used as a cathode and energized, becomes prevalent in place of an anion electrodeposition coating because metal ions are not eluted from the substance to be coated.
Since the cation electrodeposition coatings itself is superior in a rust-preventive property, they are widely used in the areas, in which a high rust-preventive property, an excellent appearance and pitting resistance are required, such as a coating system for automobiles. However, since it is difficult to thicken the coating film thickness on an edge portion of a substance to be coated, there are problems that an edge-covering property and a rust-preventive property of an edge portion are poor and improvement of them is desired.
In order to improve the edge-covering property of substances to be coated, it is considered to add an internally crosslinked fine resin particle obtained through a crosslinking reaction in a particle. It is possible to enhance the edge-covering property and to improve the rust-preventive property of an edge portion by controlling the fluidity of the coating in baking through an addition of such the internally crosslinked fine resin particle.
However, when coating is performed with a cation electrodeposition coating using such the internally crosslinked fine resin particle, there may be cases where the internally crosslinked fine resin particle is not adequately dispersed in a not-yet-cured coating film. In such a case, the rust-preventive property of an edge portion may not be adequately improved. Further, by allowing the coating to contain the internally crosslinked fine resin particle, the stability of bath of a cation electrodeposition coating, the electrodeposition properties and the water resistance of coating film may be impaired and further there was a problem that the obtained coating film was low in smoothness.